P.P.W9's and the goal of more HP

Are the seats stainless? Stainless can corrode like that if not passivated before assembly. Typically you see it under stainless dock cleats or dissimilar metals. What happens is the surface is still reactive but the assembly prevents oxygen from entering and creating the 'white layer' on the stainless. Other high alloy steels that are not stainless can behave similarly too.
Once the corrosion reaches an edge, it breaks-out like you see. Properly passivating the surface to create the 'stainless' white layer is important prior to assembly for alloys susceptible to these kinds of reactions.

Just a theory, could be wrong, but I've seen similar on other applications before.